The analysis of the different amino acid sequences deduced from the complete genome sequence of the gram-positive bacterium Staphylococcus aureus suggested the presence of two eukaryotic-protein-like low-molecular-mass phosphotyrosine protein phosphatases, which are usually found in gram-negative bacteria. To check this prediction, the corresponding genes were cloned and overexpressed in an Escherichia coli system. Two distinct proteins with an apparent molecular mass of 23 kDa each, PtpA and PtpB, were produced and then purified by affinity chromatography and assayed for enzymatic properties. As expected, they both exhibited phosphatase activity in vitro, with a maximum value at a pH of around 6.2 and at a temperature of 40 degrees C. In addition, their kinetic constants, their specificity for phosphotyrosine residues, and their sensitivity to two phosphatase inhibitors, N-ethylmaleimide and orthovanadate, matched those of acid low-molecular-mass phosphotyrosine protein phosphatases.